Thin-film transistors (also known as “organic TFT”) utilizing an organic semiconductor in an active layer (hereinafter referred to as “organic semiconductor layer”) may be mentioned as typical elements for constituting organic semiconductor devices.
In the thin-film transistors, the organic semiconductor layer is formed by forming a film of a molecular crystal typified by pentacene in vacuo. Regarding the formation of the organic semiconductor layer by the film formation in vacuo, there is a report that the optimization of the film formation conditions can realize the formation of an organic semiconductor layer having a high level of charge mobility exceeding 1 cm2/V·s (Y. -Y. Lin, D. J. Gundlach, S. Nelson, and T. N. Jackson, “Stacked Pentacene Layer Organic Thin-Film Transistors with Improved Characteristics,” IEEE Electron Device Lett. 18, 606 (1997)). The organic semiconductor layer formed by the film formation in vacuo, however, is generally in a polycrystal form composed of aggregates of fine crystals. Therefore, many grain boundaries are likely to exist, and, in addition, defects are likely to occur. The grain boundaries and defects inhibit the transfer of charges. For this reason, in the formation of the organic semiconductor layer by film formation in vacuo, the organic semiconductor layer as the element for constituting the organic semiconductor device could not have been substantially continuously produced in a satisfactory wide area with homogeneous properties without difficulties.
On the other hand, a discotic liquid crystal is known as a material having a high level of charge mobility (D. Adam, F. Closss, T. Frey, D. Funhoff, D. Haarer, H. Ringsdorf, P. Schunaher, and K. Siemensmyer, Phys. Rev. Lett., 70, 457 (1993)). In this discotic liquid crystal, however, charge transfer is carried out based on a one-dimensional charge transfer mechanism along the columnar molecular alignment. Therefore, close control of the molecular alignment is required, and this disadvantageously makes it difficult to utilize the discotic liquid crystal on a commercial scale. Any example of success in thin-film transistors using the discotic liquid crystal as a material for constituting the organic semiconductor layer has not been reported yet.
A high level of charge mobility in a liquid crystal state of rodlike liquid crystalline materials such as phenylbenzothiazole derivatives has already been reported (M. Funahashi and J. Hanna, Jpn. J. Appl. Phys., 35, L703-L705 (1996)). Up to now, however, there is no report on any example of success of thin-film transistors utilizing the rodlike liquid crystalline material in the organic semiconductor layer. The rodlike liquid crystalline material exhibits a few types of liquid crystal states, and the charge mobility is likely to increase with enhancing the regularity of the structure of the liquid crystalline material. The transition of the liquid crystalline material to a crystal state having a higher level of regularity of structure, however, results in lowered or no charge mobility. In this case, of course, no properties required of the thin-film transistor are developed.
For the utilization of the liquid crystalline material in a liquid crystal state having a high level of charge mobility, in use, the liquid crystal material should be sealed in a glass cell or the like. This imposes restrictions on device production. Further, the rodlike liquid crystalline material exhibits liquid crystallinity only at relatively high temperatures and thus cannot be utilized at a temperature around room temperature (about −10 to 40° C.).
When a molecular dispersion polymeric material is used as an organic semiconductor material, an organic semiconductor layer, which has uniform charge transfer characteristics over a large area can be formed by coating this organic semiconductor material. The organic semiconductor layer thus formed, however, has a low charge mobility of 10−5 to 10−6 cm2/V·s, and, disadvantageously, the charge mobility depends upon temperatures and electric fields.
The present inventors have solved the above problem by providing, in Japanese Patent Application No. 32772/2002, an organic semiconductor structure having, in at least a part thereof, an organic semiconductor layer comprising an aligned liquid crystalline organic semiconductor material, the liquid crystalline organic semiconductor material having a core comprising L 6 π electron aromatic rings, M 10 π electron aromatic rings, and N 14 π electron aromatic rings, wherein L, M, and N are each an integer of 0 (zero) to 4 and L+M+N=1 to 4, the liquid crystalline organic semiconductor material exhibiting at least one liquid crystal state at a temperature below the heat decomposition temperature thereof.
The above organic semiconductor structure, however, is formed from a liquid crystalline organic semiconductor material which is a nonpolymeric material. The following report is the sole report about organic semiconductor materials which are polymeric materials, and effective organic semiconductor materials have not yet been found. That is, there is no report about an organic semiconductor structure and an organic semiconductor layer having effective charge transfer characteristics.
Specifically, for example, M. Redecker and D.D.C. Bradley, Applied Physics Letters, vol. 74, 10, 1999 reports polymeric semiconductor materials having a high level of charge mobility around room temperature. According to this literature, a polymeric semiconductor material having a high level of mobility can be prepared by providing a polymer having a long conjugated system on its main chain, heating the polymer to a temperature at which a nematic phase is developed, and then rapidly cooling the polymer to form a glassy polymer with a nematic state fixed thereto. In particular, it is reported that a high level of mobility can be realized in the case where the above procedure is carried out in such a state that the material is in contact with an aligning film formed by subjecting a polyimide film to rubbing treatment.
On the other hand, at the present time, any polymeric material having a conjugated molecule on its side chain, which exhibits a high level of charge mobility, is not known. The reason for this is that, in the main chain type polymeric semiconductor, electronic conduction proceeds along the main chain having a long conjugated system, whereas, in the side chain type polymeric semiconductor, the conjugated systems in the side chain molecules overlap with each other and charges are transferred by hopping conduction between side chain molecules.
Further, any material capable of successfully controlling the overlap of side chain molecules with each other have not hitherto been known in the art, and a high level of charge mobility could not be provided. The degree of freedom in constitutive property can be improved by increasing the chain length of an alkyl group, which connects the main chain to the π conjugated aromatic ring, to increase the degree of freedom. Increasing the chain length of the alkyl group, which connects the main chain to the π conjugated aromatic ring, inevitably results in the introduction of a large amount of material not involved in electronic conduction and consequently could not contribute to a satisfactory improvement in charge mobility.
The present invention has been made with a view to solving the above problems of the prior art, and an object of the present invention is to provide an organic semiconductor material having a high level of homogenous charge mobility over a large area, which has hitherto been regarded as difficult of achieve, and to provide an organic semiconductor structure and an organic semiconductor device utilizing the organic semiconductor material.